1. Description of the invention
The present invention relates to a polythiophene-based polymer liquid composition, which can be formed into a coating with high conductivity and high transparency. More particularly, the present invention is concerned with the use of an amide solvent and a sulfonic acid group (SO.sub.3 H)-containing monomer dopant in improving the conductivity and transparency of a polythiophene-based polymer liquid composition.
2. Description of the Prior Art
Polyethylenedioxythiophene (PEDT), a conductive polymer, was disclosed as an antistatic property-imparting material, such as an antistatic coating, in U.S. Pat. Nos. 5,035,926 and 5,391,472. Since then, intensive attention has been paid to polymer-based conductive coatings applicable to glass surfaces of Braun tubes.
In terms of electrical conductivity, this conductive polymer has advantages over well-known preexisting polymers, such as polyanilines, polypyrroles and polythiophenes. PEDT also shows excellent processability by virtue of its ability to be easily formed into a coating solution which is dispersible in water, along with salts of polymer acids (e.g., polystyrene sulfonate) as dopants. With this good water dispersibility, PEDT can be well dissolved in alcohol solvents, and the aqueous dispersions find numerous applications in Braun tube (CRT) glasses, plastic film surfaces, etc. The use of water and alcohols as solvents is ecologically very favorable.
Such water-dispersible PEDT polymers are now commercially available, representatively exemplified by Baytron P (Grade A4071) from Bayer.
Highly transparent as they are, conductive PEDT polymers must be coated at low PEDT contents (24% or less based on 1.3 wt % solution) in order to exhibit a transmittance of 92%. Accordingly, it is difficult to achieve a surface resistance of 100 k.OMEGA. or less through ordinary techniques. In order to reinforce the strength of PEDT coatings, there is used silica sols prepared from alkoxy silane (e.g., alkyltriethoxysilane), represented by RSi(OR.sup.1).sub.3 wherein R is methyl, ethyl, propyl or isobutyl and R.sup.1 is methyl or ethyl. In this case, the conductivity of the resulting coatings is aggravated owing to silica sol. Thus, it is more impossible for existing techniques to produce conductive coatings with a surface resistance of 100 k.OMEGA./.quadrature. or less. In fact, current techniques can use PEDT as an antistatic coating material only where low conductivity is required (see technical reference for Baytron P, Bayer).
Therefore, it is virtually impossible for currently prevailing techniques to achieve a transmittance of 92% in addition to a surface resistance of 1 k.OMEGA./.quadrature. or less, which satisfies TCO (Tianstemanners Central Organization) Standards for commercially competitive electromagnetic wave-shielding materials.